Contact dermatitis. We don't know what material ur make up brush is made of. Most of the time contact allergy on the face is from the facial creams and make-up's themselves rather than the brush.
Brush or make-up. More likely the rash would be from an allergy to the make-up. Some people are sensitive to specific brushes, but more brushes these days are made from synthetic materials...More plastic than hair, which don't typicaly cause allergies. However, these are both treated with short courses of cool compresses, benadryl, (diphenhydramine) and if needed a little Hydrocortisone (twice a day for a week).
See below. Oral Benadryl (diphenhydramine) would be a good choice.